The present invention relates to a digital-to-analog converter (D/A converter) which utilizes semiconductor devices such as MOS transistors, and more particularly to a D/A converter which has a short conversion time and is suitable to implement in the form of an IC structure because high precision resistors are not required, in other words, to a D/A converter suitable for use in a ghost eliminator of a television receiver.
When an electromagnetic wave coming directly from a transmitting antenna (desired wave) and an electromagnetic wave reflected by a building or the like are simultaneously received by a receiving antenna, an image by the desired wave and an image by the reflected wave appear in staggered fashion so that a so-called ghost appears. Such a ghost is a large factor to degrade the image quality in the television receiver and various approaches to eliminate or prevent the ghost have been proposed in the past. One of those approaches is a ghost elimination system using a transversal filter in a video band, as disclosed in U.S. patent application Ser. No. 406,565 filed on Aug. 9, 1982 U.S. Pat. No. 4,476,491 (or EP application No. 82304216.3 filed on Oct. 8, 1982). In the disclosed ghost elimination system, a D/A converter is used to provide DC gain control voltages in accordance with digital data to a number of tapped amplifiers.
A D/A converter which comprises an integrator, a first current source, a second current source having a larger current value than that of the first current source, first connection means for connecting the first current source to the integrator for a period determined by low order bits of digital data, second connection means for connecting the second current source to the integrator for a period determined by high order bits of the digital data, and shorting means for discharging a charge stored in the integrator is disclosed in "Nikkei Electronics, page 200 published on Jan. 18, 1982." In the disclosed D/A converter, a counter is used to count the periods determined by the low order bits and the high order bits. Thus, when the digital data has ten bits which comprise five low order bits and five high order bits, a count period to count 31 clocks at maximum is required. If clock frequency is 5 MHz, the count period of 6.2 .mu.s is required. Further, a certain period is required for operation of the shorting means to discharge the load capacitance prior to the D/A conversion operation. Thus, when 100 or more tapped amplifiers are included in a transversal filter, a period of 1 ms or longer is required before tap gains are distributed to all of the amplifiers. This is a critical problem in the performance of the ghost elimination system.